Conventional air separation units (ASUs) for the production of nitrogen and oxygen by the cryogenic separation of air are basically comprised of a two-stage distillation column which operates at very low temperatures. Due to the extremely low temperatures, it is essential that water vapor and carbon dioxide be removed from the compressed air feed to an ASU. If this is not done, the low temperature sections of the ASU will freeze up making it necessary to halt production and warm the clogged sections to revaporize and remove the offending solid mass of frozen gases. This can be very costly. It is generally recognized that, in order to prevent freeze up of an ASU, the content of water vapor and carbon dioxide in the compressed air feed stream must be less than 0.1 ppm and 1.0 ppm, respectively.
A process and apparatus for the pre-purification of air must have the capacity to constantly meet, and hopefully exceed, the above levels of contamination and must do so in an efficient manner. This is particularly significant since the cost of the pre-purification is added directly to the cost of the product gases of the ASU.
Current commercial methods for the pre-purification of air include reversing heat exchangers, temperature swing adsorption and pressure swing adsorption. The first two of these approaches are described by Wilson et al in IOMA BROADCASTER, Jan.-Feb., 1984, pp 15-20.
Reversing heat exchangers remove water vapor and carbon dioxide by alternately freezing and evaporating them in their passages. Such systems require a large amount, typically 50% or more, of product gas for the cleaning, i.e. regenerating, of their passages. Therefore, product yield is limited to about 50% of feed. As a result of this significant disadvantage, combined with characteristic mechanical and noise problems, the use of reversing heat exchangers as a means of pre-purification has steadily declined over recent years.
In temperature swing adsorption (TSA) pre-purification, the impurities are removed at low temperature, typically at about 5.degree. C., and regeneration is carried out at elevated temperatures, e.g. from about 150.degree.-250.degree. C. The amount of product gas required for regeneration is typically only about 12%-15%, a considerable improvement over reversing heat exchangers. However, TSA processes require both refrigeration units to chill the feed gas and heating units to heat the regeneration gas. They are, therefore, disadvantageous both in terms of capital costs and energy consumption.
Pressure swing adsorption (PSA) processes are an attractive alternative to TSA, since both adsorption and regeneration are carried out at ambient temperature. PSA processes, in general, do require substantially more regeneration gas than TSA. This can be disadvantageous when high recovery of cryogenically separated products is required. When a PSA air pre-purification unit is coupled to a cryogenic ASU plant, a waste stream from the cryogenic section which is substantially free of water vapor and carbon dioxide is used as the regeneration gas.
Such a PSA pre-purification process is described in German patent publication DE 3,045,451 (1981). This process operates at 5.degree. to 10.degree. C., 883 KPa (9Kg/cm.sup.2) adsorption pressure and 98 KPa (1 atm) regeneration pressure. Feed air is passed under pressure through a layer of 13.times. zeolite particles to remove the bulk of water vapor and carbon dioxide and then through a layer of activated alumina particles to remove the remaining low concentrations of carbon dioxide and water vapor. Arrangement of the adsorbent layers in this manner is claimed to reduce the temperature effects, i.e. temperature drop during desorption, in the PSA beds. A process similar to that of this German patent is discussed by Tomomura et al. in KAGAKU KOGAKU RONBUNSHU. 13(5). (1987), pp 548-553. This latter process operates at 28.degree.-35.degree. C., 0.65 MPa adsorption pressure and 0.11 MPa regeneration pressure. The process has a sieve specific product of 7.1 Sm.sup.3 /min/m.sup.3 and a vent gas loss of 6.3%. The activated alumina occupies about 40% of the bed. The relative adsorbent particle sizes used are: 13.times. zeolite 2.4-4.8 mm., and activated alumina 2-4mm.
Japanese Kokai patent publication Sho 59-4414 (1984) describes a PSA pre-purification process in which separate beds and adsorbents are used for water vapor and carbon dioxide removal. The water vapor removal tower containing activated alumina or silica gel is regenerated by low pressure purge while the carbon dioxide removal tower containing 13.times. zeolite is regenerated by evacuation only without a purge. The use of a vacuum pump can be justified in some processes having a high product recovery. Regeneration gas requirements for this process (25%) are high in comparison to those of a conventional TSA pre-purification unit (PPU).
Japanese patent publication Sho 57-99316 (1982) describes a process wherein feed air, vent gas and purge gas are passed through a heat exchanger to thereby cause adsorption and desorption to take place at nearly the same temperature. The advantage of this process is stated to be a reduction in the required quantity of regeneration gas.
In the process described in Japanese patent publication Sho 55-95079 (1980), air is treated by PSA in two stages to remove water vapor and carbon dioxide wherein dry air product from the PSA unit is used to purge the first stage and an impure nitrogen stream from the ASU is used to purge the second stage. This process is stated to be advantageous in terms of the overall nitrogen recovery.
European patent publication no. 232,840 (1987) describes a PSA process utilizing activated alumina to remove water vapor and a zeolite to remove carbon dioxide. It is stated that the use of activated alumina allows removal of water vapor at a lower temperature and, therefore. adsorption of carbon dioxide takes place at a lower temperature. Both adsorption and desorption take place close to ambient temperature.
In the PSA cycle described in laid-open German Offen. DE 3,702,190 Al (1988), at least 80% of the heat of adsorption is retained in the bed and is available for regeneration. The principle of retaining heat of adsorption in PSA beds is well established in the art.
It will be appreciated that, although many pre-purification methodologies based on PSA have been proposed in the literature, few are actually being used commercially due to high capital costs associated therewith.
In general, known PSA pre-purification processes require a minimum of 25%, typically 40-50%, of the product as purge gas. As a result of having low sieve specific product, such processes have high capital cost. Reduction in the air pre-purification system capital cost is particularly important when a large plant is contemplated because scale-up for a prepurification system cost is almost linear with plant size, whereas the rest of the plant scales up by a 0.6 power law factor. Therefore, it will be readily appreciated that, for large plants, improvements in prepurification system operation can result in considerable cost savings.
In accordance with the present invention, a means of efficiently removing water vapor and carbon dioxide has been found which is advantageous over the prior art in terms of capital cost and purge gas requirement.